1. Field of the Invention
This invention relates generally to contact arrangements for single pole or multipole circuit breakers, and, more particularly, to contact arrangements that use attractive Lorentz forces between parallel conductors to open the contacts during a high current condition.
2. Description of the Prior Art
Molded case circuit breakers are generally known in the art. An example of such a circuit breaker is disclosed in U.S. Pat. No. 4,891,618, herein included by reference. Such circuit breakers are used to protect electrical circuitry from damage due to a high current condition, such as an overload, a short circuit, or both. An overload current normally is about two to three times the nominal current rating of the circuit breaker. A short circuit may produce currents that are ten or even one hundred times the nominal current rating of the circuit breaker or more.
The contact assemblies of high current circuit breakers generally include stationary, or fixed, main and arcing contacts connected to the line conductor. Movable main and arcing contacts are connected to the load conductor. The arcing contacts should make contact first and break last in order to protect the main contacts from damage due to arcing. The movable main contact is typically disposed on the bottom side of a movable first end of an elongated main contact member, or main contact arm. The second end of the main contact arm is hinged at a pivot point and connected to the load conductor. The movable arcing contact can be disposed on the underside of an arcing contact member, or arcing contact arm, pivotably attached to the first end of the main arcing contact member. In another common type of arrangements, movable contact assemblies are arranged such that a pair of main contact arms are disposed alongside the arcing contact arms.
Overload protection can be provided by a bimetal disposed in series with a load conductor. The bimetal typically consists of two strips of metal having different rates of thermal expansion and bonded together at one end. During a sustained overload the bimetal will deflect due to ohmic heating and engage the circuit breaker trip bar to trip the circuit breaker contacts.
Short circuit protection is typically provided by an electromagnet assembly, by a solid state trip unit, or by magnetic repulsion forces. Electromagnet assemblies, for example, include an electromagnet surrounding a load conductor and a cooperating armature that latches the circuit breaker trip bar during normal conditions. During a short circuit condition the short circuit current passes through the electromagnet which generates attraction forces to attract the armature and unlatch the trip bar which in turn causes the circuit breaker to trip by releasing the movable contact arm, thus opening the contacts.
Magnetic repulsion arrangements, such as disclosed in U.S. Pat. No. 4,891,618, typically consist of flexible shunts formed in generally a U-shape defining two depending legs. The flexible shunts are used to connect the pivotably mounted contact arms to the load conductors. During a short circuit condition, the current flowing in the depending legs of the shunts generates repulsion forces between the depending legs which causes the pivotably mounted contact arms to blow open. It is also well known to provide a U-shaped line side conductor carrying the fixed main and arcing contacts. In this arrangement, the current in the main contact member is directed antiparallel the direction of current flow in the nearby top leg of the U-shaped line side conductor. The oppositely directed currents produce a repulsive force that is proportional to the product of the currents and inversely proportional to the gap between the two conductors. The repulsive force lifts the main contact arm off the fixed main contact at sufficiently high currents.
The mechanisms that are currently used to trip the circuit breaker during a high current condition are very bulky. Electromagnet assemblies require large coil windings for each pole in the circuit breaker. Magnetic repulsion members, such as, for example, the flexible shunts disclosed in U.S. Pat. No. 4,891,618 and also the U-shaped line side conductor, require additional space and produce additional heat within circuit breaker housing.
Since molded case circuit breakers, and, in particular, current limiting molded case circuit breakers are relatively compact, the problem exists to provide higher current limiting capabilities for a circuit breaker in relatively smaller frame sizes. More specifically, the components in a small frame size current limiting molded case circuit breaker cannot merely be increased in size to provide an effective current limiting circuit breaker. Larger components have greater inertia and thus require larger forces to move.
Additionally, currently used short circuit trip mechanisms are relatively slow. Electromagnet assemblies generally take a full cycle to trip the breaker. While magnetic repulsion arrangements and solid state trip mechanisms are somewhat faster than electromagnet assemblies, there is a need for improvement in the speed of opening the arcing contacts after the main contacts are opened.
Further, contact assemblies that have a main contact arm with separate arcing contact arms along either side of the main contact arm can sometimes open the arcing contacts before opening the main contacts, causing premature damage to the main contacts. Therefore, there is the additional need to provide a contact arrangement that assures that the main contacts open before the arcing contacts.